1. Field of the Invention
This invention relates generally to temperature regulation and more particularly to an apparatus and method for regulating the temperature in a cryogenic test chamber to smoothly increase or decrease the temperature in the test chamber throughout its operative range, and to stably maintain any predetermined temperature within its operative range for indefinite periods of time.
2. Discussion of the Prior Art
The need for testing specimens of various types for different properties has increased substantially over the last several years. Different methods have been devised to accomplish such tests at low (cryogenic) temperatures. Temperature regulation in a cryogenic test chamber demands a sophisticated balance between supply and loss of thermal energy. A measure of the efficiency of a specific control scheme is the width of the temperature range over which control can be maintained, and the duration and stability achieved at any temperature in this range. In general, two distinct control modes of operation must be employed to achieve accurate temperature regulation above or below the normal boiling point of the liquid cryogen (refrigerant) medium used in a cryogenic vessel. This temperature is 4.2 K. for liquid helium (.sup.4 He) a frequently employed cryogen. Temperature regulation in the vicinity of the normal boiling point, however, is particularly difficult because of the large change in thermal properties (i.e. specific heat) of the liquid itself which undergoes a liquid-to-gaseous transition in this temperature region. Temperature oscillations causing thermal instability are the most common problem encountered when trying to control temperature at the normal boiling point of the cryogenic liquid and, in general, when switching between low and high temperature control modes.
When regulating temperatures above the normal boiling point of the liquid refrigerant it is common to supply heat to the test chamber to effectively maintain and control a desired temperature. Because it is rather uneconomical to use the reservoir as a direct source and then regulate the test chamber at higher temperatures, it is common to thermally isolate the test chamber from the surroundings at a lower temperature. Temperature control is then achieved by drawing a small fraction of the liquid cryogen through a passageway such as a coupling tube into the test chamber from the remainder of the cryogenic vessel maintained under a slight overpressure. The pressure difference between the test chamber and the cryogen reservoir effectively provides the sole injection mechanism of the liquid cryogen into the coupling tube and through the test chamber required for temperature control. The fluid reaching the interior of the test chamber is then heated to the desired temperature and the vapor flows through the test chamber to provide uniform temperature control. While this control scheme is suitable for high temperature regulation, its thermal response near the vicinity of the normal boiling point of the cryogenic fluid is rather poor because of the difficulty of controlling the vapor flow rate passively by the overpressure that exists in the cryogenic vessel alone.
When temperatures below the normal boiling point of the liquid refrigerant are required, a second category of methods which continuously extract heat from the test chamber is employed. In these methods the temperature of the test chamber is decreased by applying a vacuum to the vapor above the liquid phase of the cryogenic fluid, thereby decreasing its boiling point temperature. For practical applications where liquid .sup.4 He is used as the cryogenic medium, the test chamber, reaching lower temperatures than its surroundings, is again thermally isolated from the main bath in the cryogenic vessel. This is because it is very uneconomical to just pump the vapor above the liquid helium bath in the cryogenic vessel, since about 40% of the liquid must be evaporated to cool it from 4.2 to 1.5 K. due to the large change in specific heat over this temperature range. Greater efficiency is thus achieved by leaving the main part of the liquid helium at its normal boiling point and pumping only a small amount of helium previously drawn from the main bath into the test chamber. The liquid .sup.4 He accumulated in the test chamber is then evacuated to a suitable pressure such that the liquid helium boils at the desired control temperature. However, in most practical applications, because of the limited volume available in the test chamber to accumulate the cryogen in liquid form (typically 10 to 20 cubic centimeters), temperatures below the normal boiling point of the liquid .sup.4 He can be regulated only for a limited period of time, in some cases 40 to 50 minutes at the lowest temperatures. When all the liquid cryogen collected in the test chamber has been boiled away the control system must re-initiate the filling process to draw more of the liquid cryogen into the test chamber, thus interrupting effective temperature control. Only when enough liquid cryogen has been recollected in the test chamber can the desired temperature be again reached and regulated. By selecting a suitable underpressure and continuing to evacuate the test chamber as the liquid boils so as to maintain that pressure, any desired temperature down to about 1.5 K. can be reached. An example of such cryogenic test apparatus prior art device is shown in U.S. Pat. No. 4,848,093.
That patent discloses an apparatus for regulating the temperature in a test chamber in a cryogenic vessel at any temperature between about 1.5 and 300 K. by means of a controllably heated capillary tube spaced apart from the test chamber but still located within the cryogenic fluid in the cryogenic vessel. The capillary tube regulates the flow of fluid from the cryogenic vessel into the test chamber to permit any desired temperature to be achieved in the test chamber. In a high temperature mode of operation, the heated capillary tube supplies the cryogenic fluid in gaseous form and prevents any fluid in a liquid phase from entering the test chamber. In the low temperature mode, below 4.2 K., the capillary tube is heated to such an extent that very little flow of gas to the test chamber is permitted after the test chamber has been charged with liquid cryogen.
This prior art apparatus is shown here in FIG. 1, where the temperature regulation apparatus includes cryogenic vessel 11 adapted to contain a reservoir of fluid 12 in a liquid phase at a cryogenic temperature, capillary tube 13 located in the cryogenic vessel, capillary heater means 14, test chamber 15 in the cryogenic vessel but spaced apart from the capillary tube, coupling tube 16, coil 17 and coil 21 defining a fluid flow path between the capillary tube and the test chamber, test chamber heater 22, and evacuation means 38, typically comprising a pump whose throughput is regulated by a variable flow valve and appropriate pressure sensors and readout, to partially evacuate the test chamber to draw fluid from the cryogenic vessel through the capillary tube, the coupling tube and the coils into the test chamber. Appropriate microprocessor controls are employed to relate heater and vacuum pump operation to temperature readings.
The apparatus of this prior art patent regulates the temperature in the test chamber by maintaining the fluid therein at a desired temperature. In a high temperature mode, the capillary heater warms the capillary tube sufficiently to boil any liquid flowing therein, changing the phase of the fluid from liquid to gas as the fluid is drawn through the capillary tube into the test chamber. The test chamber heater warms any gas in the test chamber to the desired temperature and then the gas moves upwardly to provide uniform temperature control along or throughout the test chamber.
In a medium temperature mode (at about 4.2 K.), the fluid remains in its liquid phase without undergoing any net change in temperature as it is drawn through the capillary tube into the test chamber. The fluid is thereby maintained at the cryogenic temperature in the test chamber.
In a low temperature mode, the fluid remains in its liquid phase as it is drawn through the capillary tube into the test chamber until a reservoir of liquid has accumulated to the desired level in the test chamber. Then the capillary heater quickly warms the capillary tube sufficiently to effectively create a gas bubble or a vapor lock, thereby substantially preventing the flow of any more fluid through the capillary tube. The evacuation means thereupon reduces the pressure in the test chamber sufficiently to lower the boiling temperature of the liquid therein to the desired temperature and continues to evacuate any gas produced as the liquid boils, thereby maintaining the liquid at the desired temperature below 4.2 K.
The time required to reduce the test chamber temperature from room temperature (about 300 K.) to the low cryogenic temperature of about 1.5 K. with this prior art apparatus would be about 30 minutes in typical circumstances. Then that low temperature can only be maintained for about 30-60 minutes before refilling and recycling is necessary according to that prior art teaching.
While this apparatus worked well for many purposes, it was not possible to maintain cryogenic temperatures at a controlled level below 4.2 K. for long periods of time, for example, for several hours or even indefinitely. Some tests cannot be completed in the 30-60 minutes afforded by such prior art devices before recycling was necessary. Of course, losing the temperature stability can affect the specimen itself, resulting in inaccurate test data.
A first step which has been suggested to extend the temperature range of the cryostat is by providing a combination of a thermally isolated small capillary inlet having a suitable flow impedance and a second inlet to the test chamber reservoir regulated by a mechanical valve in the cryogenic vessel at the inlet of the temperature controlled test chamber. In this way it is possible to use a small capillary to allow continuous filling of the reservoir below 4.2 K. when the valve is closed and, in addition, obtain large flows through the second inlet to allow rapid cooling when the valve is open. Unfortunately, the difficulty of constructing reliable cryogenic valves has limited the commercial usefulness of this approach. An alternative which has been employed is to remotely locate the mechanical valve outside the cryogenic vessel. This configuration requires an inlet which is re-routed outside the cryogenic vessel through the mechanical valve.
Although this apparatus in principle allows a range of temperatures from about 1.5 K. to 400 K. to be achieved (if liquid .sup.4 He is employed), its regulation at temperatures in the vicinity of 4.2 K. is rather poor. The long inlet utilized in this configuration introduces long waiting periods of time to maintain and regulate temperatures. While this may not constitute a serious problem at temperatures above or below the boiling point of .sup.4 He, significant instabilities occur near 4.2 K. where quick thermal changes of the properties of the liquid require a fast control temperature mode.
The purposes for conducting tests of specimens at cryogenic temperatures, or at any temperature which must be maintained at some level between about 1.5 K. and 400 K., are many and varied. There may be life tests, where observations over periods of days or weeks would be accomplished if it were possible to do so. Detailed characterization of magnetic or physical properties of a new or unknown test specimens may require temperature sweeps between 15 and 400 K., or within any included sub-segments of temperature limits. Because the onset of important intrinsic physical properties of the test specimen material may occur suddenly or as a discontinuity at any temperature, it is important that temperatures be controlled smoothly as they are changed through the operating range. In addition, elaborate tests may require extended measurement time even at the lowest temperature achieved by the apparatus. For this reason it is important that temperatures be continuous, that is, that they can be maintained indefinitely. No known integrated prior art apparatus can accomplish all these purposes, that is, the ability to cycle back and forth smoothly between about 1.5 and 400 K. and to selectively indefinitely maintain any set temperature within that range.